The methylation of inorganic arsenic to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) is considered to be the major pathway for inorganic arsenic detoxification. With the development of a rapid assay for the methyltransferases of arsenic metabolism, we have investigated the liver methyltransferases of the marmoset and tamarin monkeys. We have found that the marmoset and tamarin, both New World animals, are deficient in arsenic methyltransferase activity. However, the rhesus monkey, an Old World animal, has ample amounts of such methyltransferase activities. We need to confirm these results using another marmoset and tamarin monkey liver. This is an extremely important observation and opens many holes in present theories explaining arsenic detoxification in mammalian systems.